This invention relates to antennas, and more particularly to phased array antennas.
The broad popularity and ever increasing demand for mobile communication services has given rise to many techniques and innovations geared toward increasing the capacity and communication quality of wireless networks. A wireless mobile network includes one or more base stations spread over a coverage region and a group of mobile subscribers such as cellular phone users. A base station provides a local link between the mobile subscribers and a traditional telephone line. In this context, capacity is the amount of information (i.e., number of bits) that can be exchanged in a given unit of time (or per unit bandwidth) in a given area. In more simplistic terms, the capacity of a wireless network dictates the maximum number of simultaneous wireless telephone conversations that can take place in a given geographical region.
Traditionally, subscribers communicating with the same base station have been differentiated from one another by frequency, as in FDMA (frequency division multiple access); by time, as in TDMA (time division multiple access); or by code, as in CDMA (code division multiple access). A traditional base station antenna broadcasts signals in a fixed direction covering a fixed, broad sector. In such a case, most of the energy is wasted, never reaching a subscriber. Furthermore, given a fixed, broad beam antenna, it is not possible to take advantage of the fact that subscribers in a given region are spatially diverse (i.e., their angular direction with respect to the nearest base station is different).
More recently, xe2x80x9csmart antennasxe2x80x9d have been introduced as a means of increasing capacity. For purposes of discussion herein, a xe2x80x9csmart antennaxe2x80x9d is any antenna that is capable of controlling the direction of its transmitted energy or the direction from which it receives energy. Throughout this description, it should be kept in mind that discussions relating to transmitting or transmissions apply with equal veracity to reception of electromagnetic energy or signals. In order to avoid prolixity, the present description will focus primarily on transmission characteristics of antennas, with the proviso that it is understood that reception of energy or signals is also inherently described. Smart antennas have been used in many applications over the years including phased array radar systems and in other communication systems. However, until recently, the use of xe2x80x9csmart antennasxe2x80x9d as a building block in a mobile communication network has been prohibitively expensive.
One proposed form of xe2x80x9csmart antennaxe2x80x9d is used in conjunction with digital beam forming techniques. Such an antenna includes multiple radiating columns with energy being received (or transmitted) through each column. By applying amplitude weights to the energy passing through each column the resulting radiation pattern can be very specifically tailored so that significantly more signal is transmitted or received in, or from, certain angular directions relative to other angular directions. Another way to state this is that by applying appropriate amplitude weights to the energy passing through each column, pattern nulls can be created at specific angular directions while other angular directions benefit from the full antenna gain. This ability to steer pattern nulls is useful, for example, when a given communications system is operating at what would otherwise be its full capacity.
A simplified example focusing on two subscribers competing for the same conventional channel in a TDMA system will be presented here. A conventional channel is defined as the unique combination of a carrier frequency (one of 126) and a time slot. If all conventional channels in a given sector are used up, any additional request for service must either be denied or placed on a channel that is already in use by another subscriber. Without a smart antenna, two subscribers on the same channel would immediately become interference for one another. In such a case neither would be able to communicate effectively with the base station. However, by using a null steering smart antenna it is possible to take advantage of spatial diversity and create additional channels. These additional channels are known as space division multiple access (SDMA) channels. To create such a channel the base station radio communicating with the first subscriber places a null on the second subscriber to attenuate the interfering signal. Likewise, the base station antenna communicating with the second subscriber must place a null on the first subscriber. In doing so, two independent, SDMA channels are created permitting reuse of the existing FDMA and TDMA channels. This null steering takes place very quickly (in tens of microseconds) and is synchronized with ongoing communications protocols. In this example two xe2x80x9csmart antennasxe2x80x9d located at the same base station essentially carve out two spatially discriminated channels permitting two users in the same area (or cell) to share a single conventional channel.
The principle illustrated in the above example is simplified in order to facilitate understanding the invention, and fails to address some of the realities that may significantly limit the utility of adding null steering capability to a mobile communication network. One such limiting reality is signal spreading. Signal spreading refers to the fact that the received communication signal in a mobile network may arrive at a base station (for example) from many different angles. Signal spreading results because radio frequency energy naturally follows all available reflection paths between a mobile subscriber and a base station (this is commonly referred to as xe2x80x9cmulti-pathxe2x80x9d propagation, or simply xe2x80x9cmulti-pathxe2x80x9d). To make matters worse, the angle from which the strongest signal arrives is not necessarily constant and, in fact, may change very rapidly as a function of time. Thus, because of signal spreading, a single spatially narrow null will not always provide enough isolation to create an independent channel. Furthermore, even if a spatial channel can be created at times, it is not possible to guarantee that the channel can be maintained continuously. This is because the signal-spreading signature changes rapidly as subscribers move and as other objects in the physical channel move. Given enough angular separation between signals, it may be possible to detect and compensate for all of these changes in real time and realize spatially independent channels. However, in general, multi-path effects will tend to significantly limit this otherwise theoretically clean method of significantly improving capacity using SDMA. And in any event, the null steering and angle of arrival algorithms required to actually realize capacity improvements are very computationally intense. Such algorithms are very costly to develop, require significant hardware upgrades for implementation in today""s networks, and also require significant computer resources.
High costs and other uncertainties associated with digital beam forming have provided impetus for developing lower cost measures for exploiting spatial subscriber diversity. One such measure includes dividing a broad sector into multiple fixed sectors. An independent fixed antenna with a narrower beam is then used to service each of the smaller sectors. Each sectored beam originates from an independent aperture at the base station. An antenna with a narrower beam has more gain and is not as susceptible to interference as a broad beam antenna. Thus, the use of a narrow beam improves both the signal to noise ratio and the signal to interference ratio (sometimes referred to as xe2x80x9ccarrier to interference ratioxe2x80x9d in telecommunication systems) within the network. In most cases, mobile communication networks are interference limited. Thus, an improved signal to interference ratio can be exploited to increase network capacity. A typical mobile communications network is divided into many cells with frequencies being re-used among cells. Use of the same frequencies in adjacent cells is avoided to avoid interference between cells. Two cells are able to re-use the same frequency they are sufficiently geographically separated. This is the case because as radio frequency energy propagates through the atmosphere it becomes attenuated. Such attenuation is known as propagation loss. After a sufficient distance the power in an interference signal from one cell is guaranteed to be low enough so that it does not create a problem in the cell of interest. Narrow, sectored beams inherently experience less interference. This means that less allowance for propagation loss is needed in frequency planning in order to guarantee an adequate signal to interference ratio. If less allowance for propagation loss is needed, more frequencies are available for reuse among cells, and planning to avoid inter-cell interference is less stringent so that frequency reuse among cells is more easily accommodated. The focused nature of narrow beam systems also means that separations among frequencies in a particular cell may be reduced without contributing to interference between adjacent frequencies. This translates into what is known as a more dense frequency re-use pattern and increases overall network capacity. The increased capacity arises because a fixed number of conventional frequency channels can be re-used more times in the same geographical region.
The use of fixed, sector beam antennas is certainly more cost effective than antennas based on digital beam forming. However, because the beams are fixed, their potential to improve quality and capacity is limited. The beam width of a fixed antenna cannot adapt to changing conditions as would otherwise be required to maintain an optimal signal to interference ratio. Furthermore, a fixed beam provides less gain to a given subscriber who happens to be positioned near the edge of the beam. This is the case because, by definition, antenna gain decreases by 3 dB at the beam edges.
Sectored antennas are cost effective but their ability to exploit spatial diversity is limited because the associated beams are fixed. The digital beam-forming antenna described above operates by radiating energy over a wide angular sector and placing nulls at specific points to minimize unwanted interferers. Multiple nulls are often required, and each null must then be steered to track the interfering subscribers. Steering multiple nulls in real time leads to a high degree of computational intensity. An alternate approach to null steering is to radiate energy in a focused, scanning beam with no nulls, pointing the beam directly at the subscriber to be communicated with. In such a narrow scanning beam system, the beam itself is steered to track a subscriber instead of using a broad fixed beam and steering nulls to track interfering subscribers. A narrow scanning beam system offers two principal advantages. First, because the beam is focused, it has higher gain. This is advantageous because it increases signal power (thereby increasing the signal to noise ratio) creating a more robust communication channel. Second, because each narrow beam antenna must track the location of no more than one subscriber at a time, the computational intensity is reduced as compared with a broad fixed beam system.
A significant reason for the growing interest in smart antennas is the potential for increased capacity and quality. In densely populated areas mobile systems are normally interference-limited, meaning that interference from other users is the main source of noise in the system. This means that the signal to interference ratio, SIR, is much larger than the signal to noise-ratio, SNR. Smart antennas (by employing a narrow focused beam) operate to increase the useful received signal level while also reducing adverse effects of interference. A smart antenna based on a narrow, scanning beam is more directive than a conventional wide beam antenna and this contributes to increased range. Increased range means that base stations can be spaced further apart leading to reductions in total deployment costs. This is particularly advantageous in emerging markets where new infrastructure is still being created. Still another advantage of smart antennas is the possibility of introducing additional services. A smart antenna provides the base station with a means of determining subscriber position. Positioning can be used in services such as emergency calls and location-specific billing.
The communication quality provided by a mobile network can be indicated using several measures. An important measure is clarity of communication (i.e., the percentage of a conversation during which two people communicating over the network understand each other). Another measure is the frequency at which communication is unintentionally interrupted or terminated.
This invention seeks to provide antennas for use in mobile communications networks that can improve the communication quality provided by the networks.
An electronic scanning antenna system configured for beam scanning operation includes a plurality of antenna modules, each respective antenna module including: (a) a plurality of antenna elements arranged in a plurality of element sets, the plurality of element sets being arranged in a plurality of columns; (b) a plurality of beam forming network devices coupled with the plurality of antenna elements, each respective beam forming network device including at least one tunable dielectric phase shifter unit; and (c) at least one control unit coupled with the plurality of beam forming network devices, each control unit controlling the plurality of beam forming network devices to configure signals to operate the plurality of antenna elements to effect the beam scanning operation.
An electronic scanning antenna includes a plurality of columns of radiating elements for forming a plurality of beams, a beam forming network including a plurality of tunable dielectric phase shifters coupled to the radiating elements, and a controller for controlling a phase shift of the tunable dielectric phase shifters to adjust a beam pointing angle of at least one of the plurality of beams and to adjust a beam width of at least one of the plurality of beams.
The tunable dielectric phase shifters are preferably mounted in a plurality of manifolds with each of the manifolds being coupled to a plurality of the columns of radiating elements. A plurality of low noise amplifiers can be included, with each of the low noise amplifiers being connected between each column of radiating elements and the plurality of manifolds. The antenna can further include a plurality of band pass filters, each of the band pass filters being connected in series with each low noise amplifier.
The controller can include a direction of arrival processor, a beam pointing/beam width processor, and a phase shift controller for producing a phase shift control signal in response to a signal produced by the direction of arrival processor and the beam pointing/beam width controller. The direction of arrival processor produces a control signal in response to a signal to interference ratio produced by a base station receiver. The beam pointing/beam width processor produces a control signal for pointing the beam and for controlling the beam width in response to a framing/frequency-hopping schedule generated by the base station receiver and in response to a direction of arrival signal from the direction of arrival processor.
The controller further includes means for supplying control voltages to the phase shifters. The control voltages control the phase shift provided by the phase shifters as required for beam scanning and beam width control. Beam scanning and beam width control are then used by the direction of arrival processor to maximize the signal to interference ratio. The radiating elements can comprise dual polarized radiating elements to further enhance signal to interference ratios by exploiting polarization diversity.
The invention also encompasses an electronic scanning antenna system comprising a first plurality of columns of radiating elements for forming a first plurality of beams, a first beam forming network including a first plurality of tunable dielectric phase shifters coupled to the first plurality of columns of radiating elements, a first controller for controlling a phase shift of the first plurality of tunable dielectric phase shifters to adjust a beam pointing angle of at least one of the first plurality of beams and to adjust a beam width of at least one of the first plurality of beams, a second plurality of columns of radiating elements for forming a second plurality of beams, a second beam forming network including a second plurality of tunable dielectric phase shifters coupled to the second plurality of columns of radiating elements, and a second controller for controlling a phase shift of the second plurality of tunable dielectric phase shifters to adjust a beam pointing angle of at least one of the second plurality of beams and to adjust a beam width of at least one of the second plurality of beams.
The first controller can comprise means for monitoring a signal to interference ratio of a received signal, and means for supplying control voltages to the first plurality of phase shifters to control a phase shift provided by those phase shifters to maximize the signal to interference ratio.
The invention also encompasses a multi-beam antenna comprising a plurality of one dimensional, electronically scanning apertures, each of the apertures including a plurality of radiating elements receptive to both vertically polarized and horizontally polarized radio frequency energy, an independent bank of tunable dielectric phase shifters coupled to the radiating elements for each polarization, an independent combining/dividing network for each polarization, and an independent voltage control circuit for each tunable dielectric phase shifter.
The apertures can include means for connection to a neighboring aperture. Each of the apertures in the array can operate simultaneously while any other set of apertures in the same array is operating. The antenna can operate in the 1 to 2 GHz frequency band servicing mobile communications subscribers.
Each of the apertures can utilize a substantially planar topology and incorporate dielectrically tunable phase shifters. Each of the apertures can include a combining/dividing network comprising radio frequency circuitry on a plurality of closely stacked, vertically disposed layers.
The dielectrically tunable phase shifters, control circuitry for the phase shifters, and the voltage control lines connecting the phase shifters to the control circuitry can be arranged substantially on the same plane as the associated combining/dividing network, or on one or more planes of a multi-layer structure closely spaced apart from the associated combining/dividing network. In other words, the electronic control circuitry can be integrated with the RF circuitry on the same physical carrier medium.
The invention additionally encompasses a multi-beam antenna comprising a plurality of electronically scanning apertures, each of the apertures including a plurality of radiating elements, an independent bank of dielectrically tunable phase shifters, and at least one combining/dividing network, and an independent voltage control circuit for each dielectrically tunable phase shifter, an independent central processing unit for computing phase shifter control commands as required to realize electronic beam steering.
In another aspect, the invention additionally encompasses a multi-beam antenna with centralized beam computational hardware comprising a plurality of one dimensional, electronically scanning apertures, each of the apertures including a plurality of radiating elements receptive to both vertically polarized and horizontally polarized radio frequency energy, an independent bank of dielectrically tunable phase shifters for each polarization, an independent combining/dividing network for each polarization, an independent voltage control circuit for each dielectrically tunable phase shifter, and an independent amplifier for each column of radiating elements. The amplifiers provide independent amplitude control of the radio frequency energy propagating through each column of the aperture.
Antennas constructed in accordance with this invention can also include an antenna controller comprising means for monitoring the signal to interference ratio of received signals, means for electronically steering the antenna beam, means for electronically adjusting the antenna beam width, a feedback loop that automatically adjusts the beam pointing angle of the antenna as required to maximize the signal to interference ratio of a received signal, and a feedback loop that automatically adjusts the beam width of the antenna as required to maximize the signal to interference ratio of a received signal.
A plurality of isolated combining/dividing networks can be connected in series with the antenna aperture. An independent set of electronically tunable phase shifters can be connected in series with each combining-dividing network. In addition, independent control circuitry can be provided for each phase shifter.
The antenna can further include a plurality of radiating elements receptive to both vertically polarized and horizontally polarized radio frequency energy, an independent combining/dividing network for each polarization, and an independent set of phase shifters in series with each combining dividing network.
Each aperture can be substantially planar in topology and can incorporate dielectrically tunable phase shifters. The antenna can further include combining/dividing networks comprised of radio frequency circuitry on a plurality of closely stacked, vertically disposed layers.
The control circuitry for the phase shifters and the voltage control lines connecting the phase shifters to the control circuitry can share a common carrier with an associated combining/dividing network.